1. Lectures

2. Operating System: An OS is a program that controls the execution of application programs and acts as an interface between the user of a computer and the computer hardware. 3. Operating System Services Program creation Program execution Access to I/O devices Controlled access to files System access Error detection and response Accounting 4. The Operating System (OS) typically provides services in the following areas: Program creation: The OS provides a variety of facilities and services, such as editors and debuggers, to assist the programmer in creating programs. Typically, these services are in the form of utility programs that are not actually part of the OS but are accessible through the OS. Program execution: A number of tasks need to be performed to execute a program Instructions and data must be loaded into main memory, I/O devices and files must be initialized, and other resources must be prepared. The OS handles all of this for the user. Access to I/O devices: Each I/O device requires its own specific set of instructions or control signals for operation. The OS takes care of the details so that the programmer can think in terms of simple reads and writes. 5. Controlled access to files: In the case of files, control must include an understanding of not only the nature of the I/O device (disk drive, tape drive) but also the file format on the storage medium. Again, the OS worries about the details. Further, in the case of a system with multiple simultaneous users, the OS can provide protection mechanisms to control access to the files. System access: In the case of a shared or public system, the OS controls access to the system as a whole and to specific system resources.The access function must provide protection of resources and data from unauthorized users and must resolve conflicts for resource contention. 6. Error detection and response: A variety of errors can occur while a computer system is running.These include internal and external hardware errors, such as a memory error, or a device failure or malfunction; and various software errors, such as arithmetic overflow, attempt to access forbidden memory location, and inability of the OS to grant the request of an application. In each case, the OS must make the response that clears the error condition with the least impact on running applications.The response may range from ending the program that caused the error, to retrying the operation, to simply reporting the error to the application. 7. Accounting: A good OS collects usage statistics for various resources and monitor performance parameters such as response time. On any system, this information is useful in anticipating the need for future enhancements and in tuning the system to improve performance. On a multiuser system, the information can be used for billing purposes. 8. A computer is a set of resources for the movement, storage, and processing of data and for the control of these functions. The OS is responsible for managing these resources. 9. Layers and Views of a Computer System 10. Types of Operating System Interactive Batch Single program (Uni-programming) Multi-programming (Multi-tasking) 11. . An interactive system, the user/ programmer interacts directly with the computer, usually through a keyboard/display terminal, to request the execution of a job or to perform a transaction. A batch system is the opposite of interactive. The users program is batched together with programs from other users and submitted by a computer operator. After the program is completed, results are printed out for the user. 12. Multiprogramming, keeps the processor as busy as possible, by having it work on more than one program at a time. Several programs are loaded into memory, and the processor switches rapidly among them. The alternative is a uniprogramming system that works only one program at a time 13. Early Systems Late 1940s to mid 1950s No Operating System Programs interact directly with hardware Two main problems: Scheduling Setup time 14. These early systems presented two main problems.: Scheduling: Most installations used a sign-up sheet to reserve processor time. Typically, a user could sign up for a block of time in multiples of a half hour or so. A user might sign up for an hour and finish in 45 minutes; this would result in wasted computer idle time. On the other hand, the user might run into problems, not finish in the allotted time, and be forced to stop before resolving the problem. Setup time: A single program, called a job, could involve loading the compiler plus the high-level language program (source program) into memory, saving the compiled program (object program), and then loading and linking together the object program and common functions. Each of these steps could involve mounting or dismounting tapes, or setting up card decks. If an error occurred, the hapless user typically had to go back to the beginning of the setup sequence. Thus a considerable amount of time was spent just in setting up the program to run. 15. Simple Batch Systems Resident Monitor program Users submit jobs to operator Operator batches jobs Monitor controls sequence of events to process batch When one job is finished, control returns to Monitor which reads next job Monitor handles scheduling 16. Simultaneous Peripheral Operation Online, or SPOOLing, is the simplest form of multiprogramming. 17. Multiprogramming systems: extend the idea of spooling and batch processing to allow several executing programs to be in memory concurrently. This is achieved by cycling through processes, allowing each one to use the CPU for a specific slice of time. Monitors were able to handle multiprogramming to a certain extent and could start jobs, spool operations, perform I/O, switch between user jobs, and give some protection between jobs. 18. Time Sharing Systems Allow users to interact directly with the computer i.e. Interactive Multi-programming allows a number of users to interact with the computer 19. In a timesharing system, the CPU switches between user sessions very quickly, giving each user a small slice of processor time. This procedure of switching between processes is called context switching. The operating system performs these context switches quickly, in essence, giving the user a personal virtual machine. Timesharing permits many users to share the same CPU. By extending this idea, a system can allow many users to share a single application. As with timesharing systems, large interactive system users are unaware of the other users on the system. 20. Memory Management Uni-program Memory split into two One for Operating System (monitor) One for currently executing program Multi-program User part is sub-divided and shared among active processes 21. Swapping Problem: I/O is so slow compared with CPU that even in multi-programming system, CPU can be idle most of the time Solutions: Increase main memory Expensive Leads to larger programs Swapping 22. What is Swapping? Long term queue of processes stored on disk Processes swapped in as space becomes available As a process completes it is moved out of main memory If none of the processes in memory are ready (i.e. all I/O blocked) Swap out a blocked process to intermediate queue Swap in a ready process or a new process But swapping is an I/O process 23. Use of Swapping 24. Partitioning Splitting memory into sections to allocate to processes (including Operating System) Fixed-sized partitions May not be equal size Process is fitted into smallest hole that will take it (best fit) Some wasted memory Leads to variable sized partitions 25. Real-time, Multiprocessor, and Distributed/Networked Systems Real-time systems are used for process control in manufacturing plants, assembly lines, robotics etc. Real-time systems have severe timing constraints. If specific deadlines are not met, physical damage or other undesirable effects to persons or property can occur. Because these systems must respond to external events, correct process scheduling is critical. In hard real-time systems (with potentially fatal results if deadlines aren't met), there can be no errors. In soft real-time systems, meeting deadlines is desirable, but does not result in catastrophic results if deadlines are missed. 26. Multiprocessor systems have more than one processor that must be scheduled. In multiprocessing environment, the CPUs cooperate with each other to solve problems, working in parallel to achieve a common goal. System synchronization requirements determine whether the processors are designed using tightly coupled or loosely coupled communication methods. Tightly coupled multiprocessors share a single centralized memory, which requires that an operating system must synchronize processes very carefully to assure protection. Symmetric multiprocessors (SMPs) are a popular form of tightly coupled architecture. These systems have multiple processors that share memory and I/O devices. All processors perform the same functions, with the processing load being distributed among all of them. 27. Loosely coupled multiprocessors have a physically distributed memory, also known as distributed systems. A distributed collection of workstations on a LAN, each with its own operating system, is typically referred to as a networked system. These systems were motivated by a need for multiple computers to share resources. A network operating system includes the necessary provisions, such as remote command execution, remote file access, and remote login, to attach machines to the network. Network file systems are one of the most important applications of networked systems. These allow multiple machines to share one logical file system, although the machines are located in different geographical locations and may have different architectures and unrelated operating systems. 28. Operating System Design The operating system controls the basic functions of the computer, including memory management and I/O, not to mention the "look and feel" of the interface. An operating system differs from most other software in that it is event driven, meaning it performs tasks in response to commands, application programs, I/O devices, and interrupts. Four main factors drive operating system design: performance, power, cost, and compatibility. 29. Two components are crucial in operating system design: the kernel and the system programs. The kernel is the core of the operating system and is used by the process manager, the scheduler, the resource manager, and the I/O manager and is responsible for scheduling, synchronization, protection/security, memory management, and dealing with interrupts. It has primary control of system hardware, including interrupts, control registers, status words, and timers, loads all device drivers, provides common utilities, and coordinates all I/O activity. The kernel must know the specifics of the hardware to combine all of these pieces into a working system. 30. Operating System Services The operating system oversees all critical system management tasks, including memory management, process management, protection, and interaction with I/O devices and the operating system determines how the user interacts with the computer, serving as a buffer between the user and the hardware. Each of these functions is an important factor in determining overall system performance and usability. 31. The Human Interface The operating system provides a layer of abstraction between the user and the hardware of the machine. The operating system provides an interface to hide the details of the bare machine. Operating systems provide three basic interfaces, each providing a different view for a particular individual. Hardware developers are interested in the operating system as an interface to the hardware. Applications developers view the operating system as an interface to various application programs and services. Ordinary users are most interested in the graphical interface, which is the interface most commonly associated with the term interface 32. Operating system user interfaces can be divided into two general categories: command line interfaces graphical user interfaces (GUIs). Command line interfaces provide a prompt at which the user enters various commands, including those for copying files, deleting files, providing a directory listing, and manipulating the directory structure. GUIs provide a more accessible interface for the casual user. They include features such as icons and other graphical representations of files that are manipulated using a mouse. 33. Process Management Process management includes everything from creating processes (setting up the appropriate structures to store information about each one), to scheduling processes' use of various resources, to deleting processes and cleaning up after their termination. The operating system keeps track of each process, its status (which includes the values of variables, the contents of CPU registers, and the actual staterunning, ready, or waitingof the process), the resources it is using, and those that it requires. 34. The kernel is managing a collection of processes, consisting of user processes and system processes. Process scheduling is a large part of the operating system's normal routine. Multitasking (allowing multiple processes to run concurrently) and multithreading (allowing a process to be subdivided into different threads of control) provide interesting challenges for CPU scheduling. 35. A thread is the smallest schedulable unit in a system. Threads share the same execution environment as their parent process, including its CPU registers and page table. Depending on the degree of concurrency required, it is possible to have one process with one thread, one process with multiple threads, multiple single-threaded processes, or multiple multithreaded processes. An operating system that supports multithreading must be able to handle all combinations 36. Resource Management In addition to process management, the operating system manages system resources. For example, multiple processes can share one processor, multiple programs can share physical memory, and multiple users and files can share one disk. There are three resources that are of major concern to the operating system: the CPU, memory, and I/O. Access to the CPU is controlled by the scheduler. Memory and I/O access requires a different set of controls and functions 37. Security and Protection Resource sharing, however, creates a multitude of exposures, such as the potential for unauthorized access or modification of data. Concurrent processes must be protected from each other, and operating system processes must be protected from all user processes. Without this protection, a user program could potentially wipe out the operating system code for dealing with, say, interrupts. Multiuser systems require additional security services to protect both shared resources (such as memory and I/O devices) and nonshared resources (such as personal files). Memory protection safeguards against a bug in one user's program affecting other programs or a malicious program taking control of the entire system. CPU protection makes sure user programs don't get stuck in infinite loops, consuming CPU cycles needed by other jobs. 38. Positional Numbering The general idea behind positional numbering systems is that a numeric value is represented through increasing powers of a radix (or base). This is often referred to as a weighted numbering system because each position is weighted by a power of the radix. The set of valid numerals for a positional numbering system is equal in size to the radix of that system. For example, there are 10 digits in the decimal system, 0 through 9, and 3 digits for the ternary (base 3) system, 0, 1, and 2. The largest valid number in a radix system is one smaller than the radix, so 8 is not a valid numeral in any radix system smaller than 9. To distinguish among numbers in different radices, we use the radix as a subscript, such as in 3310 to represent the decimal number 33. 39. Example: Three numbers represented as powers of a radix. 243.5110 = 2 x 102 + 4 x 101 + 3 x 100 + 5 x 10-1 + 1 x 10-2 2123 = 2 x 32 + 1 x 31 + 2 x 30 = 2310 101102 = 1 x 24 + 0 x 23 + 1 x 22 + 1 x 21 + 0 x 20 = 2210 40. Signed Integer Representation Signed numbers require additional issues to be addressed. When an integer variable is declared in a program, many programming languages automatically allocate a storage area that includes a sign as the first bit of the storage location. By convention, a "1" in the high-order bit indicates a negative number. The storage location can be as small as an 8-bit byte or as large as several words, depending on the programming language and the computer system. The remaining bits (after the sign bit) are used to represent the number itself. How this number is represented depends on the method used. There are three commonly used approaches. The most intuitive method, signed magnitude, uses the remaining bits to represent the magnitude of the number 41. Signed Magnitude Up to this point, we have ignored the possibility of binary representations for negative numbers. The set of positive and negative integers is referred to as the set of signed integers. The problem with representing signed integers as binary values is the signhow should we encode the actual sign of the number? Signedmagnitude representation is one method of solving this problem. As its name implies, a signed-magnitude number has a sign as its left- most bit (also referred to as the high-order bit or the most significant bit) while the remaining bits represent the magnitude (or absolute value) of the numeric value. For example, in an 8-bit word, 1 would be represented as 10000001, and +1 as 00000001. In a computer system that uses signed-magnitude representation and 8 bits to store integers, 7 bits can be used for the actual representation of the magnitude of the number. This means that the largest integer an 8-bit word can represent is 27 1 or 127 (a zero in the high-order bit, followed by 7 ones). The smallest integer is 8 ones, or 127. Therefore, N bits can represent 2(N 1) 1 to 2(N 1) 1. 42. Complement Systems Number theorists have known for hundreds of years that one decimal number can be subtracted from another by adding the difference of the subtrahend from all nines and adding back a carry. This is called taking the nine's complement of the subtrahend, or more formally, finding the diminished radix complement of the subtrahend. Let's say we wanted to find 167 52. Taking the difference of 52 from 999, we have 947. Thus, in nine's complement arithmetic we have 167 52 = 167 + 947 = 114. The "carry" from the hundreds column is added back to the units place, giving us a correct 167 52 = 115. This method was commonly called "casting out 9s" and has been extended to binary operations to simplify computer arithmetic. The advantage that complement systems give us over signed magnitude is that there is no need to process sign bits separately, but we can still easily check the sign of a number by looking at its high-order bit. 43. Another way to envision complement systems is to imagine an odometer on a bicycle. Unlike cars, when you go backward on a bike, the odometer will go backward as well. Assuming an odometer with three digits, if we start at zero and end with 700, we can't be sure whether the bike went forward 700 miles or backward 300 miles! The easiest solution to this dilemma is simply to cut the number space in half and use 001500 for positive miles and 501999 for negative miles. We have, effectively, cut down the distance our odometer can measure. But now if it reads 997, we know the bike has backed up 3 miles instead of riding forward 997 miles. The numbers 501999 represent the radix complements (the second of the two methods introduced below) of the numbers 001500 and are being used to represent negative distance. 44. Error Detection And Correction Regardless of the coding method used, no communications channel or storage medium can be completely error-free. It is a physical impossibility. As transmission rates are increased, bit timing gets tighter. As more bits are packed per square millimeter of storage, flux densities increase. Error rates increase in direct proportion to the number of bits per second transmitted, or the number of bits per square millimeter of magnetic storage. A parity bit could be added to an ASCII byte to help determine whether any of the bits had become corrupted during transmission. This method of error detection is limited in its effectiveness: Simple parity can detect only an odd number of errors per byte. If two errors occur, we are helpless to detect a problem. 45. you should keep in mind that just as it is impossible to create an error-free medium, it is also impossible to detect or correct 100% of all errors that could occur in a medium. Error detection and correction is yet another study in the tradeoffs that one must make in designing computer systems. The well-constructed error control system is therefore a system where a "reasonable" number of the "reasonably" expected errors can be detected or corrected within the bounds of "reasonable" economics. 46. Cyclic Redundancy Check Checksums are used in a wide variety of coding systems, from bar codes to International Standard Book Numbers (ISBNs). These are self- checking codes that will quickly indicate whether the preceding digits have been misread. Cyclic redundancy check (CRC) is a type of checksum used primarily in data communications that determines whether an error has occurred within a large block or stream of information bytes. The larger the block to be checked, the larger the checksum must be to provide adequate protection. Checksums and CRCs are a type of systematic error detection scheme, meaning that the error-checking bits are appended to the original information byte. The group of error-checking bits is called a syndrome. The original information byte is unchanged by the addition of the error-checking bits. The word cyclic in cyclic redundancy check refers to the abstract mathematical theory behind this error control system. Although a discussion of this theory is beyond the scope of this text, we can demonstrate how the method works to aid in your understanding of its power to economically detect transmission errors. 47. Arithmetic Modulo 2 You may be familiar with integer arithmetic taken over a modulus. Twelve-hour clock arithmetic is a modulo 12 system that you use every day to tell time. When we add 2 hours to 11:00, we get 1:00. Arithmetic modulo 2 uses two binary operands with no borrows or carries. The result is likewise binary and is also a member of the modulus 2 system. Because of this closure under addition, and the existence of identity elements, mathematicians say that this modulo 2 system forms an algebraic field. The addition rules are as follows: 0 + 0 = 0 0 + 1 = 1 1 + 0 = 1 1 + 1 = 0 48. Example: Find the sum of 10112 and 1102 modulo 2. 1011 +110 11012(mod 2) This sum makes sense only in modulo 2. 49. Instruction Sets: Characteristics and Functions The operation of the processor is determined by the instructions it executes, referred to as machine instructions or computer instructions. The collection of different instructions that the processor can execute is referred to as the processors instruction set. 50. Instruction Cycle State Diagram 51. Elements of a Machine Instruction Each instruction must contain the information required by the processor for execution. The figure above shows the steps involved in instruction execution and, by implication, defines the elements of a machine instruction which are: Operation code: Specifies the operation to be performed (e.g., ADD, I/O). The operation is specified by a binary code, known as the operation code, or opcode. Source operand reference: The operation may involve one or more source operands, that is, operands that are inputs for the operation 52. Elements Result operand reference: The operation may produce a result. Next instruction reference: This tells the processor where to fetch the next instruction after the execution of this instruction is complete 53. The address of the next instruction to be fetched could be either a real address or a virtual address, depending on the architecture. Source and result operands can be in one of these areas: Main or virtual memory: As with next instruction references, the main or virtual memory address must be supplied. Processor register: With rare exceptions, a processor contains one or more registers that may be referenced by machine instructions. If only one register exists, reference to it may be implicit. If more than one register exists, then each register is assigned a unique name or number, and the instruction must contain the number of the desired register. Immediate: The value of the operand is contained in a field in the instruction being executed. I/O device: The instruction must specify the I/O module and device for the operation. If memory-mapped I/O is used, this is just another main or virtual memory address. 54. Instruction Representation Opcodes are represented by abbreviations, called mnemonics, that indicate the operation. Common examples include ADD Add SUB Subtract MUL Multiply DIV Divide LOAD Load data from memory STOR Store data to memory Operands are also represented symbolically. For example, the instruction ADD R, Y may mean add the value contained in data location Y to the contents of register R. In this example, Y refers to the address of a location in memory, and R refers to a particular register. Note that the operation is performed on the contents of a location, not on its address. 55. Types of Operands Machine instructions operate on data which have these general categories: Addresses Numbers Characters Logical data Other common data types are numbers, characters, and logical data, and some machines define specialized data types or data structures. For example, there may be machine operations that operate directly on a list or a string of characters. 56. Numbers All machine languages include numeric data types. Even in nonnumeric data processing, there is a need for numbers to act as counters, field widths. The distinction between numbers used in ordinary mathematics and numbers stored in a computer is that the latter are limited which is true in two ways. First, there is a limit to the magnitude of numbers representable on a machine and second, in the case of floating-point numbers, a limit to their precision 57. Numbers Three types of numerical data are common in computers: Binary integer or binary fixed point Binary floating point Decimal 58. Intel x86 and ARM Data TypesThe x86 can deal with data types of 8 (byte), 16 (word), 32 (doubleword), 64 (quadword), and 128 (double quadword) bits in length. To allow maximum flexibility in data structures and efficient memory utilization, words need not be aligned at evennumbered addresses; doublewords need not be aligned at addresses evenly divisible by 4; and quadwords need not be aligned at addresses evenly divisible by 8, and so on. However, when data are accessed across a 32-bit bus, data transfers take place in units of doublewords, beginning at addresses divisible by 4. 59. The processor converts the request for misaligned values into a sequence of requests for the bus transfer. The byte, word, doubleword, quadword, and double quadword are referred to as general data types Nb: ARM is a RISC design that relies heavily on register addressing 60. Types of Data and descriptions General Byte: word (16 bits), doubleword (32 bits), quadword (64 bits), and double quadword (128 bits) locations with arbitrary binary contents. Integer: A signed binary value contained in a byte, word, or doubleword, using twos complement representation. Ordinal: An unsigned integer contained in a byte, word, or doubleword. Unpacked binary coded decimal (BDC): A representation of a BCD digit in the range 0 through 9, with one digit in each byte. Packed BCD : Packed byte representation of two BCD digits; value in the range 0 to 99. Near pointer: A 16-bit, 32-bit, or 64-bit effective address that represents the offset within a segment. Used for all pointers in a nonsegmented memory and for references within a segment in a segmented memory. 61. Far pointer : A logical address consisting of a 16-bit segment selector and an offset of 16, 32, or 64 bits. Far pointers are used for memory references in a segmented memory model where the identity of a segment being accessed must be specified explicitly. Bit field : A contiguous sequence of bits in which the position of each bit is considered as an independent unit. A bit string can begin at any bit position of any byte and can contain up to 32 bits. Bit string: A contiguous sequence of bits, containing from zero to 2^32-1 bits. Byte string: A contiguous sequence of bytes, words, or doublewords, containing from zero to 2^32-1 bytes. Floating point Packed SIMD (single instruction, multiple data): Packed 64-bit and 128-bit data types 62. Intel x86 data types The packed SIMD (single-instruction-multiple-data) data types were introduced to the x86 architecture as part of the extensions of the instruction set to optimize performance of multimedia applications which included MMX (multimedia extensions) and SSE (streaming SIMD extensions). The basic concept is that multiple operands are packed into a single referenced memory item and that these multiple operands are operated on in parallel. 63. The data types are as follows: Packed byte and packed byte integer: Bytes packed into a 64-bit quadword or 128-bit double quadword, interpreted as a bit field or as an integer Packed word and packed word integer: 16-bit words packed into a 64-bit quadword or 128-bit double quadword, interpreted as a bit field or as an integer Packed doubleword and packed doubleword integer: 32-bit doublewords packed into a 64-bit quadword or 128- bit double quadword, interpreted as a bit field or as an integer. Packed quadword and packed qaudword integer: Two 64-bit quadwords packed into a 128-bit double quadword, interpreted as a bit field or as an integer Packed single-precision floating-point and packed double-precision floatingpoint: Four 32-bit floating- point or two 64-bit floating-point values packed into a 128- bit double quadword 64. Types of OperationsThe number of different opcodes varies widely from machine to machine. However,the same general types of operations are found on all machines. A useful and typical categorization is the following: Data transfer Arithmetic Logical Conversion I/O System control Transfer of control 65. Some common Arithmetic Operations and meanings ADD : Perform the arithmetic sum of two operands SUBTRACT : Perform the arithmetic difference of two operands MULTIPLY: Perform the product of two operands DIVIDE: Perform the division of two operands INCREMENT: Add one to the contents of a register DECREMENT: Subtract one from the contents of a register 66. Some Common Logical Operations and meanings AND: Perform the logical ANDing of two operands OR: Perform the logical ORing of two operands EXOR: Perform the XORing of two operands NOT: Perform the complement of an operand COMPARE: Perform logical comparison of two operands and set flag accordingly SHIFT: Perform logical shift (right or left) of the content of a register ROTATE: Perform logical shift (right or left) with wraparound of the content of a register 67. Some Transfer of Control Operations and meaning BRANCH-IF-CONDITION: Transfer of control to a new address if condition is true JUMP : Unconditional transfer of control CALL: Transfer of control to a subroutine RETURN: Transfer of control to the caller routine 68. Examples of Condition Flags Negative (N): Set to 1 if the result of the most recent operation is negative, it is 0 otherwise Zero (Z): Set to 1 if the result of the most recent operation is 0, it is 0 otherwise Overflow (V): Set to 1 if the result of the most recent operation causes an overflow, it is 0 otherwise Carry (C) : Set to 1 if the most recent operation results in a carry, it is 0 otherwise 69. ArithmeticMost machines provide the basic arithmetic operations of add, subtract, multiply, and divide. These are invariably provided for signed integer (fixed-point) numbers. Often they are also provided for floating-point and packed decimal numbers. Other possible operations include a variety of single- operand instructions like: Absolute: Take the absolute value of the operand. Negate: Negate the operand. Increment: Add 1 to the operand. Decrement: Subtract 1 from the operand 70. Intel x86 and ARM operation types The x86 provides a complex array of operation types, including a number of specialized instructions.The intent was to provide tools for the compiler writer to produce optimized machine language translation of high-level language programs. Typical operations fall in the following categories: Data movement Arithmetic Logical Control transfer String operations High level language support Flag control Segment register Protection Cache management 71. presentation Mandatory: explain the following operations Data movement Arithmetic Logical Control transfer String operations High level language support Flag control Segment register Protection Cache management 72. Assembly Language A CPU can understand and execute machine instructions. An assembly language program is composed of : Constants Expressions Literals Reserved Words Mnemonics Identifiers Directives Instructions Comments 73. Integer Constants Integer constants can be written in decimal, hexadecimal, octal or binary, by adding a radix (or number base) suffix to the end . Radix Suffices: d decimal (the default) h hexadecimal q or o octal b binary 74. Integer Expressions An integer expressions is a mathematical expressions involving integer values and integer operators. The expressions must be one that can be stored in 32 bits (or less). The precedence: ( ) Expressions in Parentheses +, - Unary Plus and minus *, /, Mod Multiply, Divide, Modulus +, - Add, Subtract 75. Real Number Constants There are two types of real number constants: Decimal reals, which contain a sign followed by a number with decimal fraction and an exponent: [sign] integer.[integer][exponent] Examples: +3.0 -44.2E+05 26.E5 Encoded reals, which are represented exactly as they are stored: 3F80000r 76. Character Constants A character constant is a single character enclosed in single or double quotation marks. The assembler converts it to the equivalent value in the binary code ASCII: A d 77. String Constants A string constant is a string of characters enclosed in single or double quotation marks: ABC x Goodnight, Gracie 4096 This isnt a test Say Goodnight, Gracie 78. Reserved words Reserved words have a special meaning to the assembler and cannot be used for anything other than their specified purpose. They include: Instruction mnemonics Directives Operators in constant expressions Predefined symbols such as @data which return constant values at assembly time. 79. Identifiers Identifiers are names that the programmer chooses to represent variables, constants, procedures or labels. Identifiers: can have 1 to 247 characters are not case-sensitive begin with a letter , underscore, @ or $ and can also contain digits after the first character. cannot be reserved words 80. Directives Directives are commands for the assembler, telling it how to assemble the program. Directives have a syntax similar to assembly language but do not correspond to Intel processor instructions. Directives are also case-insensitive: Examples .data .code name PROC 81. Instructions An instruction in Assembly language consists of a name (or label), an instruction mnemonic, operands and a comment The general form is: [name] [mnemonic] [operands] [; comment] Statements are free-form; i.e, they can be written in any column with any number of spaces between in each operand as long as they are on one line and do not pass column 128. 82. Labels Labels are identifiers that serve as place markers within the program for either code or data. These are replaced in the machine-language version of the program with numeric addresses. We use them because they are more readable: mov ax, [9020] vs. mov ax, MyVariable 83. Instruction Mnemonics Instruction mnemonics are abbreviations that identify the operation carried out by the instruction: mov - move a value to another location add - add two values sub - subtract a value from another jmp - jump to a new location in the program mul - multiply two values call - call a procedure